x分之1-y分之1=4
(xy)分之(y-x)=4
y-x=4xy
x-y=-4xy
(x-2xy-y)分之(2x+xy-2y)
=[(x-y)-2xy]分之[2(x-y)+xy]
=(-4xy-2xy)分之(-8xy+xy)
=(-6xy)分之(-7xy)
=6分之7